Many industrial operations, particularly mining operations, use large off-highway vehicles. In recent years, the size of these vehicles has grown dramatically. Currently, the largest off-highway trucks can have payloads of four-hundred tons or more and the empty weight of these vehicles can exceed three-hundred tons. In view of the size of these vehicles, the expense to manufacture and purchase them is also significant. Accordingly, in order to recoup costs of such vehicles as quickly as possible, they are often operated continuously twenty-four hours per day. The constant operation of these vehicles, coupled with sometimes harsh environmental conditions can be challenging and it is not unusual for an off-highway truck or vehicle to occasionally become disabled. Because of the sheer size and weight of such off-highway trucks or vehicles the task of recovering and/or retrieving such a disabled off-highway truck or vehicle and moving it to a repair facility can be unique and challenging.
Off-highway trucks in particular are one of the most prevalent mobile vehicles on a typical mine site. By way of reference, 90 to 95% availability of a mine off-highway truck fleet is considered excellent availability, with 5% to 10% of the off-highway truck fleet being unavailable or disabled at any time. Accordingly, in a fleet of fifty off-highway trucks, at any given time, five trucks might be disabled and in need of repairs. Assuming that there are five disabled trucks at a given time, it is reasonable to expect that at least two or three of these trucks becomes disabled at some distance from the site's maintenance and repair facility.
The typical way to move disabled off-highway trucks is to either tow them with chains or cables or pick them up by the front bumper to get the off-highway truck's front tires off the ground and then pull on the front bumper to move the off-highway truck and/or pick them up at the rear preferably near the rear axle to get the off-highway truck's rear tires off the ground and then pull on this rear attachment point to move the off-highway truck. To understand the forces required to lift the front of such an off-highway truck, consider a two hundred fifty ton capacity off-highway truck. The ‘empty’ weight of such an off-highway truck is about two hundred fifty thousand pounds. The vertical force required to lift the front of this vehicle, when empty, is about ninety thousand pounds. To understand the forces required to lift the rear of such an off-highway truck, consider a two hundred fifty ton capacity off-highway truck. The ‘empty’ weight of such an off-highway truck is about two hundred fifty thousand pounds. The vertical force required to lift the rear of this vehicle, when empty, is about one hundred thousand pounds. In contrast, if the vehicle is in the ‘loaded’ condition, this off-highway truck can weigh about eight hundred fifty thousand pounds and the force required to lift the front bumper would be about one hundred eighty thousand pounds and the force required to lift the rear axle would be about five hundred sixty thousand pounds. Thus the preferred approach is, where possible, to pull a disabled off highway truck from the front.
In some mines, it is typical to use a dedicated off-highway truck towing chassis with an incorporated lifting hook extending rearward from the off-highway truck towing chassis to hook on and lift/pull the disabled vehicles. The rearward extended lifting hook must extend far enough behind the rear tires of the towing chassis so that the rear tires of the towing chassis do not come into contact with the front of the disabled vehicle as the pair of vehicles move or turn. Thus, this lifting arrangement requires an off-highway truck towing chassis that is heavy enough to counterbalance the load imparted on the lifting hook when one end (e.g., the front end) of the disabled vehicle is picked up. Accordingly, it is often necessary to counterweight the front bumper of the towing chassis. Consequently, when not towing a disabled vehicle, the front axle of the off-highway truck towing chassis is often near overload or overloaded. Also, when the off-highway truck towing chassis has lifted and is towing a disabled vehicle, the rear axle of such an off-highway truck towing chassis is carrying an extreme load and may tend to be overloaded.
Accordingly there are disadvantages associated with the conventional off-highway truck towing chassis that uses the above-described towing hook. One major disadvantage is the rear axle loading of the off-highway truck towing chassis that occurs when towing a disabled vehicle. The entire load being picked up and towed is being carried on the rear axle of the off-highway truck towing chassis. Another significant disadvantage is that a front bumper counter balance weight is normally required on the off-highway truck towing chassis. These two factors severely impact the off-highway truck towing chassis, i.e., the rear axle loading when towing a disabled vehicle and the counterbalanced front axle load weight when not towing a vehicle, which may often result in extreme wear on the off-highway truck towing chassis at the front and rear chassis axles. Another drawback is the mechanical disadvantage at which the towing or lifting arrangement operates. When operating from the rear of the off-highway truck towing chassis, the size of the hydraulic cylinders and hydraulic oil flow required to raise and lower this lifting/towing hook is large. A further issue is the inability to easily move the lifting or towing hook sideways as it aligns with a companion socket on the disabled vehicle's bumper. An additional drawback to the conventional towing arrangement is the sheer structural size required to operate at a considerable distance behind the rear tires of the off-highway truck towing chassis.
In view of the foregoing a need has developed to provide a superior lifting and towing arrangement for all sizes of off-highway vehicles and in particular for disabled off-highway trucks that are either empty or loaded.